[Aavso-photometry] Correcting for differences in colour between comparison and program star during differential photometry

[Lionel Catalan]

While doing differential photometry, I've noticed that the calculated
magnitude of some check stars tends to drift slightly upward or downward
with time as the air mass increases. This effect correlates well with the
difference in colour (B-V) between the check star and the comparison star.
Obviously, the program star must be also affected by changes in air mass
when its colour is not exactly equal to that of the comparison star. I am
wondering whether there is an accepted procedure to correct for this effect.
It seems to me that in principle, one should be able to determine the
parameters necessary for this correction by doing photometry on a standard
field containing stars of different colours over a range of air masses.
However, I haven't been able to find such a procedure clearly outlined
either in the AAVSO ccd photometry manual or other resources available on
the web. Does such a procedure exist? (please note that I'm not trying to do
all sky photometry. I'm just trying to improve on my differential photometry
results).

 

Lionel Catalan